/*
 * @(#)ECFieldF2m.java	1.5 10/03/23
 *
 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
package java.security.spec;

import java.math.BigInteger;
import java.util.Arrays;

/**
 * This immutable class defines an elliptic curve (EC)
 * characteristic 2 finite field.
 *
 * @see ECField
 *
 * @author Valerie Peng
 * @version 1.5, 03/23/10
 *
 * @since 1.5
 */
public class ECFieldF2m implements ECField {

    private int m;
    private int[] ks;
    private BigInteger rp;

    /**
     * Creates an elliptic curve characteristic 2 finite
     * field which has 2^<code>m</code> elements with normal basis.
     * @param m with 2^<code>m</code> being the number of elements.
     * @exception IllegalArgumentException if <code>m</code>
     * is not positive.
     */
    public ECFieldF2m(int m) {
	if (m <= 0) {
	    throw new IllegalArgumentException("m is not positive");
	}
	this.m = m;
	this.ks = null;
	this.rp = null;
    }

    /**
     * Creates an elliptic curve characteristic 2 finite
     * field which has 2^<code>m</code> elements with 
     * polynomial basis.
     * The reduction polynomial for this field is based
     * on <code>rp</code> whose i-th bit correspondes to
     * the i-th coefficient of the reduction polynomial.<p> 
     * Note: A valid reduction polynomial is either a 
     * trinomial (X^<code>m</code> + X^<code>k</code> + 1
     * with <code>m</code> > <code>k</code> >= 1) or a
     * pentanomial (X^<code>m</code> + X^<code>k3</code> 
     * + X^<code>k2</code> + X^<code>k1</code> + 1 with
     * <code>m</code> > <code>k3</code> > <code>k2</code> 
     * > <code>k1</code> >= 1). 
     * @param m with 2^<code>m</code> being the number of elements.
     * @param rp the BigInteger whose i-th bit corresponds to
     * the i-th coefficient of the reduction polynomial. 
     * @exception NullPointerException if <code>rp</code> is null.
     * @exception IllegalArgumentException if <code>m</code> 
     * is not positive, or <code>rp</code> does not represent 
     * a valid reduction polynomial. 
     */
    public ECFieldF2m(int m, BigInteger rp) {
	// check m and rp
        this.m = m;
        this.rp = rp;
        if (m <= 0) {
            throw new IllegalArgumentException("m is not positive");
        }
	int bitCount = this.rp.bitCount();
	if (!this.rp.testBit(0) || !this.rp.testBit(m) ||
	    ((bitCount != 3) && (bitCount != 5))) {
	    throw new IllegalArgumentException
		("rp does not represent a valid reduction polynomial");
	}
	// convert rp into ks
	BigInteger temp = this.rp.clearBit(0).clearBit(m);
	this.ks = new int[bitCount-2];
	for (int i = this.ks.length-1; i >= 0; i--) {
	    int index = temp.getLowestSetBit();
	    this.ks[i] = index;
	    temp = temp.clearBit(index);
	}
    }

    /**
     * Creates an elliptic curve characteristic 2 finite
     * field which has 2^<code>m</code> elements with
     * polynomial basis. The reduction polynomial for this
     * field is based on <code>ks</code> whose content
     * contains the order of the middle term(s) of the 
     * reduction polynomial. 
     * Note: A valid reduction polynomial is either a
     * trinomial (X^<code>m</code> + X^<code>k</code> + 1
     * with <code>m</code> > <code>k</code> >= 1) or a
     * pentanomial (X^<code>m</code> + X^<code>k3</code>
     * + X^<code>k2</code> + X^<code>k1</code> + 1 with
     * <code>m</code> > <code>k3</code> > <code>k2</code>
     * > <code>k1</code> >= 1), so <code>ks</code> should
     * have length 1 or 3.
     * @param m with 2^<code>m</code> being the number of elements. 
     * @param ks the order of the middle term(s) of the
     * reduction polynomial. Contents of this array are copied 
     * to protect against subsequent modification.
     * @exception NullPointerException if <code>ks</code> is null.
     * @exception IllegalArgumentException if<code>m</code> 
     * is not positive, or the length of <code>ks</code> 
     * is neither 1 nor 3, or values in <code>ks</code> 
     * are not between <code>m</code>-1 and 1 (inclusive) 
     * and in descending order. 
     */
    public ECFieldF2m(int m, int[] ks) {
	// check m and ks
        this.m = m;
        this.ks = (int[]) ks.clone();
	if (m <= 0) {
	    throw new IllegalArgumentException("m is not positive");
	}
	if ((this.ks.length != 1) && (this.ks.length != 3)) {
	    throw new IllegalArgumentException
		("length of ks is neither 1 nor 3");
	}
	for (int i = 0; i < this.ks.length; i++) {
	    if ((this.ks[i] < 1) || (this.ks[i] > m-1)) {
		throw new IllegalArgumentException
		    ("ks["+ i + "] is out of range");
	    }
	    if ((i != 0) && (this.ks[i] >= this.ks[i-1])) {
		throw new IllegalArgumentException
		    ("values in ks are not in descending order");
	    }
	}
	// convert ks into rp
	this.rp = BigInteger.ONE;
	this.rp = rp.setBit(m);
	for (int j = 0; j < this.ks.length; j++) {
	    rp = rp.setBit(this.ks[j]);
	}
    }
 
    /**
     * Returns the field size in bits which is <code>m</code>
     * for this characteristic 2 finite field.
     * @return the field size in bits.
     */
    public int getFieldSize() {
	return m;
    }

    /**
     * Returns the value <code>m</code> of this characteristic
     * 2 finite field.
     * @return <code>m</code> with 2^<code>m</code> being the 
     * number of elements.
     */
    public int getM() {
	return m;
    }
 
    /**
     * Returns a BigInteger whose i-th bit corresponds to the 
     * i-th coefficient of the reduction polynomial for polynomial 
     * basis or null for normal basis. 
     * @return a BigInteger whose i-th bit corresponds to the 
     * i-th coefficient of the reduction polynomial for polynomial
     * basis or null for normal basis.
     */
    public BigInteger getReductionPolynomial() {
	return rp;
    }
 
    /**
     * Returns an integer array which contains the order of the 
     * middle term(s) of the reduction polynomial for polynomial 
     * basis or null for normal basis.
     * @return an integer array which contains the order of the 
     * middle term(s) of the reduction polynomial for polynomial 
     * basis or null for normal basis. A new array is returned 
     * each time this method is called.
     */
    public int[] getMidTermsOfReductionPolynomial() {
	if (ks == null) { 
	    return null; 
	} else {
	    return (int[]) ks.clone();
	}
    }
 
    /**
     * Compares this finite field for equality with the
     * specified object. 
     * @param obj the object to be compared.
     * @return true if <code>obj</code> is an instance
     * of ECFieldF2m and both <code>m</code> and the reduction 
     * polynomial match, false otherwise.
     */
    public boolean equals(Object obj) {
	if (this == obj) return true;
	if (obj instanceof ECFieldF2m) {
	    // no need to compare rp here since ks and rp 
  	    // should be equivalent
	    return ((m == ((ECFieldF2m)obj).m) &&
		    (Arrays.equals(ks, ((ECFieldF2m) obj).ks)));
	}
	return false;
    }
 
    /**
     * Returns a hash code value for this characteristic 2 
     * finite field.
     * @return a hash code value.
     */
    public int hashCode() {
	int value = m << 5;
	value += (rp==null? 0:rp.hashCode());
	// no need to involve ks here since ks and rp 
	// should be equivalent.
	return value;
    }
}
